The present invention relates to instruments for producing musical sounds by electronic means. In particular, this invention is concerned with instruments of the type which produce sounds by generating voltages or currents corresponding to frequencies and amplitudes of notes played one at a time. This class of instruments, called monophonic synthesizers, is popular for their versatility in musical expression and for the unnatural sounds which they can produce for special effects. The invention is, however, versatile, and may be applied to polyphonic (organ-type) synthesizers.
Most often the monophonic synthesizer contains a keyboard of the organ-type which controls the frequency (pitch) of the note being played; however some synthesizers have provision for external control of frequency and amplitude, such as by electrical signals from a microphone. Since the monophonic synthesizer produces but one note at a time, its design usually contains one main frequency generator which produces a tone at the fundamental frequency of the note struck. Typically this generator is a voltage-controlled oscillator (VCO) designed to provide one or more types of waveforms having rich harmonic content, such as square waves, triangle waves, sawtooth waves, and pulses. Sine waves (pure tones having no harmonics) are usually not produced because it is difficult to generate a low distortion sine wave over a wide frequency range and a single pure tone is musically uninteresting. Thus the variety of tonal qualities (timbres) possible from present synthesizers is limited to those which can be derived from filtering the harmonic-rich VCO outputs. This limitation has been compensated to some degree by developing versatility in voltage envelope and pitch control which cannot be duplicated by other instruments, such as polyphonic organs and conventional wind and string instruments. But even their limited application as musical instruments, present-day synthesizers lack the frequency accuracy and stability necessary for ensemble performances and for the now popular technique of multi-track recording of a single instrument on repeated playing, for an ensemble effect. Further, present techniques for vibrato generation in both organs and synthesizers do not automatically provide for adjusting the vibrato amplitude (frequency swing) according to the note played, thus creating a disparity in aural vibrato effect between low and high notes. Conventional musical instruments, on the other hand are played such that the vibrato represents a nearly constant musical interval about the note being played.
Accordingly, it is an object of this invention to provide an instrument which can produce musical sounds having a wide variety of timbres encompassing those of all known instruments, wind and string, without complex filtering of pulse or ramp waveforms.
It is also an object of this invention to provide an instrument which maintains a preset timbre over the compass of at least five octaves without a corresponding variation in volume.
It is another object of this invention to provide keyboard control of fundamental pitch and prevent simultaneous key depressions from deleteriously affecting instrument output, while using a single switch contact for each key.
It is another object of this invention to provide an instrument having versatile frequency and amplitude envelope control at high signal levels without introducing appreciable distortion.
It is yet another object of this invention to provide an instrument which will accept the electrical signal produced by an external agency, separate frequency and amplitude components, and generate sounds of preset timbre whose pitch and volume are proportional to the signal received.